MORPHOLOGY OF THE ORTHOID SHELL 35 



divisible into two parts: (1) the shajt or anterior element, and (2) the myofhore or posterior part, 

 the seat of attachment of the diductor muscles. The shaft is usually extended forward for some dis- 

 tance or may merge with a median ridge. Rarely is the shaft much abbreviated (R/iipidomella). 

 The head or myophore may be expanded, multilobate, and crenulated, or it may be compressed 

 laterally so that the actual muscular attachment takes place at the thinned posterior extremity of the 

 process (see pi. 1 7, fig. 26). In some of the later genera, such as Schizophoria, the process, which in 

 young shells has much the same form as in R/iipidomella, 



becomes absorbed and thus narrowed with age making a thin and sharp ridge; concomitant with this change is 

 the formation, in the delthyrial [notothyrial] cavity, of one, two or even three minor ridges on each side of the 

 original process, so that in old shells the posterior face of the process appears to be multilobate.'" 



Accessory ridges are also present in some species of Schizoramtna (see pi. 5, fig. 14). 

 The following nine types of cardinal process have been observed in the Orthacea: 

 (1) The Orthis type, which consists of a thin ridge situated in the middle of the notothyrial 

 platform and usually extending its whole length. This ridge is generally thicker at its base; it 

 may or may not be expanded toward the front. Scars of muscular attachment may actually be 



Fig. 9. — Diagram showing features of the cardinalia. My, myophore of the cardinal process; sh, shaft of same; Den, 

 denticle on outside wall of socket, which articulates with accessory socket in ventral tooth ; SA, so called secondary area. 



observed in some genera on the ridge, but they do not appear to be restricted to it, since in some 

 species (see pis. 2 and 5) distinct muscle marks can be observed on the notothyrial platform at the 

 base of the process. The development of a vertical ridge naturally increases the area of muscle 

 attachment and so strengthens the pulling power of the diductor muscles. The Orthis type of cardi- 

 nal process is the most primitive structurally and the earliest chronologically. Its first appearance 

 is in the Middle Cambrian and it characterizes the Billingsellidas, Orthids, and Clitambonitida:. So 

 far as our knowledge goes, this type of cardinal process is absent in punctate Orthoidea. A few punc- 

 tate genera (Harknessellinas, Heterorthinx) have a cardinal process that simulates the Orthis type, 

 but in all examples it can be shown to be a modified lobate or Dalmanella type. 



(2) The Productorthis type (see pi. 3, fig. 11; pi. 4, fig. 1 5), a unique and very unusual varia- 

 tion from the ordinary kinds seen in the Orthoidea, which has been developed as an adaptation to the 

 productoid form of the shell. The dorsal valve is nearly flat and the cardinal process rises abruptly 

 from a posterior thickening of the shell and is directed posteriorly, extending for some distance 

 beyond the margin of the hinge-line. The shaft is strong and thick, the myophore a compressed 

 region on the postero-dorsal face of the shaft, clearly showing the diductor scars. 



(3) The Dinorthis type (see pi. 9, figs. 3, 20), characteristic of the Dinorthidas, in which the 

 shaft is short and stout. In mature shells having this type of cardinal process, the median ridge 

 appears not to be a continuation of the shaft of the process but is given off rather from the anterior 

 end of the notothyrial platform. The shaft is subelliptical or suboval in section. In young mature 

 shells the myophore is likewise oval or elliptical in plan, usually dilated slightly laterally, and 

 clearly divisible into a right and left half l)y a more or less elevated ridge (Pla-siomys) or depression 

 (Dinorthis) running along the long axis of the ellipse. Each half ellipse is strongly wrinkled by 

 arcuate ridges running backwards to the margin of the myophore. 



In shells having this type of cardinal process, age modifications introduce notable changes in 

 size and shape. In PliFsiomys the shaft may be totally obliterated by enlargement of the notothyrial 



'"Hall and Clarke, Pal. N. Y., vol. 8, pt. I, p. 212. 



